In recent years, a high-voltage electrical storage device having high energy density has been desired as a power supply (drive power supply) for driving an electronic device. In particular, a lithium-ion battery and a lithium-ion capacitor have been expected to be a high-voltage electrical storage device having high energy density.
A further reduction in size has been desired for such a high-voltage electrical storage device having high energy density. In order to reduce the size of the electrical storage device, it is necessary to reduce the thickness of the separator that isolates the positive electrode and the negative electrode, in addition to the thickness of the positive electrode, the negative electrode, and the like, for example. However, a short circuit may easily occur when the gap between the positive electrode and the negative electrode decreases along with a reduction in size of the electrical storage device.
In particular, when the electrical storage device utilizes metal ions such as lithium ions, dendrites tend to be produced on the surface of the electrode due to the metal ions during repeated charge and discharge. Such dendrites normally precipitate in the form of a needle-like crystal, and are easily grown through the separator (i.e., porous membrane). If dendrites have been grown through the separator, and have reached the surface of the other electrode, the electrical storage device is short-circuited, and the charge-discharge function is lost. It is likely that such a phenomenon occurs, and reliability deteriorates along with a reduction in thickness of the separator and a reduction in the gap between the positive electrode and the negative electrode.
For example, WO2009/041395 and JP-A-2009-87562 disclose a technique that improves the battery characteristics by forming a porous layer that includes a resin binder including a polyamide, polyimide, or polyamideimide on the porous separator substrate in order to prevent the above phenomenon. JP-A-2009-54455 discloses a technique that improves the battery characteristics by forming a porous protective film that includes a binder including a fluorine-based resin and a rubber-based resin on at least one of the surface of the positive electrode and the surface of the negative electrode. WO2010/074202 discloses a technique that improves the battery characteristics by forming a porous layer that includes a copolymer including a (meth)acrylonitrile monomer unit and a (meth)acrylate monomer unit on the porous separator substrate.